Signal distortion limits the sensitivity and bandwidth of any communication system. A form of distortion commonly referred to as intersymbol interference (ISI) is manifested in the temporal spreading and consequent overlapping of individual pulses, or “symbols.” Severe ISI prevents receivers from distinguishing symbols and consequently disrupts the integrity of received signals.
The quality of high-speed signals is largely dependent upon characteristics of the communication channel. In extreme cases, ISI imposed by the channel renders the received data entirely unintelligible. Transmitters and receivers are therefore equipped with various forms of equalizers designed to offset channel-induced distortion. These equalizers are commonly adjustable to account for differences between channels.
Receivers in high-speed communication systems sometimes include control circuitry that monitors various characteristics of incoming signals and tunes the associated equalization circuitry accordingly. Such equalization adjustments may be done once, to account for channel characteristics and process variations, or may be carried out continuously or periodically to additionally account for time-variant parameters, such as supply voltage and temperature. Applying such adjustments to an equalizing transmitter requires receivers to communicate back to the associated transmitter, a process sometimes referred to as “backchannel” communication.
Backchannel communication takes place in the direction opposite the flow of data, and so may require one or more additional signal paths and associated pins between the communicating circuits. Unfortunately, as circuit features grow smaller, the number of physical pads that fits on the surface of a semiconductor die becomes a limiting factor on the amount of logic instantiated on a given circuit die. It is therefore undesirable to provide dedicated pins in support of backchannel communication.
Many communication systems support bidirectional communication, in which case backchannel communication in support of a forward channel can be incorporated into a return channel. For example, in a system in which first and second integrated circuits are connected via forward and return paths, control bits for the transmitter on the first integrated circuit can be inserted into the return path. Unfortunately, this option potentially reduces the communication bandwidth of the return path and requires the integrated circuits adopt a modified communication protocol. There is therefore a desire for improved support for backchannel communication. More generally, there is a desire to support additional communication channels within and between integrated circuits without the need for additional pads and associated conductors.